The figure shows the micturition reflex. When the urine is released, which structure relaxes while the muscle of the bladder contracts?

Fig. 1: The micturition reflex with letters. P is the kidney on the left. Q is the thin tube that leaves it. R is the box at the top that receives the signal from the bladder and sends back the motor signal. S marks the small green receptors in the wall of the bladder. T marks the ring of muscle at the start of the tube that leaves the bladder. A dashed arrow runs from the bladder to R, and solid arrows run from R to the bladder muscle and to T.